1. Field of the Invention
The present invention relates to a diffraction grating, an optical pick-up for focusing a laser beam passing through the diffraction grating to an optical disc, and an error signal detection device and an error signal detection method for detecting for example a tracking error signal or other error signal.
2. Description of the Related Art
In an optical disc drive, when an optical disc is tilted, a signal quality of a recording signal and/or reproduced signal of the optical disc may be lowered. When correcting this tilt of the optical disc, it is necessary to detect the tilt of the optical disc and generate a signal (tilt error signal) in response to the detected tilt.
U.S. Pat. No. 5,936,923 discloses an optical pick-up having a laser beam source, an object lens, and a liquid crystal panel for correcting aberration, and changing a refractive index of the liquid crystal panel in response to the thickness or tilt angle of the optical disc.
U.S. Pat. No. 5,936,926 also discloses the detection of tilt angle by a tilt sensor and the drive of the liquid crystal panel by a liquid crystal panel control circuit based on the detected tilt angle for changing the refractive index thereof.
When accessing at a predetermined address region on a track of an optical disc by an optical pick-up, it is necessary to generate a track traverse, (cross track) signal. This cross track signal is a signal electrically shifted in phase from a tracking error signal by 90 degrees, and is spatially shifted by exactly xc2xc of a pitch of the tracks or track guide grooves.
In order to move an optical spot from the optical pick-up to a predetermined target track, it is necessary to detect a speed of movement and a direction of movement of the optical spot. In an optical disc drive, in order to detect the direction of movement of the optical spot, generally the tracking error signal and the cross track signal are used.
The optical disc drive performs a tracking servo control based on the tracking error signal.
As the method of detection (or method of generation) of the tracking error signal, there are known a one spot push-pull method for generating the signal from an output signal of a photo detector by utilizing one optical spot, a differential pushpull (DPP) method for generating the signal from output signals of the photo detector utilizing three optical spots, and so on.
FIGS. 1A and 1B are views illustrating an enlarged recording surface of an optical disc and a circuit for generating a tracking error signal and other signals. In the optical disc drive, for example, the optical pick-up focuses the laser beam on the recording surface of the optical disc to generate an optical spot M0 and receives the laser beam reflected at the recording surface to generate various signals, such as a track error signal, a focus error signal and an RF signal.
FIG. 1A is a view of the recording surface of an optical disc. Lands LA and grooves GR are formed on the recording surface. The pitch of the lands LA and the pitch of the grooves GR are equal to a track pitch Tp. The center of the optical spot M0 is positioned at the center of the land LA of the recording surface. This optical spot M0 moves in a radial direction R.
FIG. 1B is a view illustrating a circuit for generating a tracking error signal or other signal based on the laser beam reflected at the recording surface (reflected laser beam).
The light receiving portion 15S of a two-divided photo detector receives a laser beam reflected at the recording surface (or the optical spot M0 on the recording surface) whereby an optical spot M10 is formed. The optical spot M10 moves in the direction R corresponding to the radial direction R when the optical spot M0 on the recording surface moves in the radial direction R. The light receiving portion 15S is equally divided into two along a direction corresponding to the track direction and has first and second light reception regions 15A and 15B.
The first light reception region 15A generates a light reception signal S15A in accordance with the laser beam irradiated on the region 15A.
The second light reception region 15B generates a light reception signal S15B in accordance with the laser beam irradiated on the region 15B.
A subtractor 101 generates a tracking error signal TE10 (=S15Axe2x88x92S15B) as the push-pull signal by subtracting the light reception signal S15B from the light reception signal S15A.
An adder 102 generates a sum signal RF10 (=S15A+S15B) by adding the light reception signals S15A and S15B. This sum signal (reproduced signal) RF10 corresponds to the amount of reflected light of the laser beam.
A high pass filter (HPF) 103 generates a cross track signal CT10 by extracting an alternating current component (or high frequency component) of the sum signal RF10.
FIG. 2 is a schematic waveform diagram illustrating the tracking error signal TE10 and the sum signal RF10, generated in the circuit in FIG. 1.
The sum signal RF10 becomes the maximum value when the center of the optical spot M0 is positioned at the center of the land LA and becomes the minimum value when the center of the optical spot M0 is positioned at the center of the groove GR.
By eliminating a direct current component DCr from this sum signal RF10 (for example eliminating the direct current component DCr by extracting the alternating current component by the HPF), the cross track signal CT10 can be obtained.
The tracking error signal TE10 becomes 0 in signal level when the center of the optical spot M0 is positioned at the center of the land LA and when it is positioned at the center of the groove GR, while becomes the maximum value or minimum value when the center of the optical spot M0 is positioned at a border between the land LA and the groove GR.
FIGS. 3A and 3B are views illustrating the recording surface of the optical disc and the light receiving portion of the photo detector when generating the cross track signal. In the optical disc drive, for example, the optical pick-up generates a laser beam comprised of a 0-th order diffraction light and xc2x11st order diffraction light by a diffraction grating, focuses the laser beams via an object lens to the recording surface of the optical disc to form three optical spots M1, S1, and S2, and receives the laser beams reflected at the recording surface by the photo detector to generate various signals.
FIG. 3A is a view of the recording surface of the optical disc. Lands LA and grooves GR are formed on the recording surface. The pitch of the lands LA and the pitch of the grooves GR are equal to the track pitch Tp. The center of the main optical spot M1 is positioned at the center of the land LA, and the centers of the sub optical spots S1 and S2 are positioned at borders between lands LA and the groove GR. The main optical spot M1 is positioned at the middle of the sub optical spots S1 and S2. The optical spots M1, S1, and S2 move in the radial direction R of the optical disc.
FIG. 3B is a view illustrating light receiving portions of the photo detector receiving the laser beams reflected at the recording surface.
A main light receiving portion 16S0 and sub light receiving portions 16S1 and 16S2 of the photo detector are irradiated with the laser beams reflected at the recording surface, whereby main optical spot M11, first and second sub optical spots S11 and S12 are formed. The optical spots M11, S11, and S12 move in the direction R corresponding to the radial direction R when the optical spots M1, S1, and S2 on the recording surface move in the radial direction R of the optical disc.
The main light receiving portion 16S0 is irradiated with the laser beam reflected at the recording surface of the optical disc (or the main optical spot M1 on the recording surface), whereby the main optical spot M11 is formed. This main light receiving portion 16S0 is equally divided into two along a direction corresponding to the track direction and has first and second light reception regions 16A and 16B. The first light reception region 16A generates a signal in accordance with the laser beam irradiated to the region 16A. The second light reception region 16B generates a signal in accordance with the laser beam irradiated to the region 16B.
The sub light receiving portion 16S1 is irradiated with the laser beam reflected at the recording surface of the optical disc (or the sub optical spot S1 on the recording surface), whereby the sub optical spot S11 is formed. This sub light receiving portion 16S1 is equally divided into two along a direction corresponding to the track direction and has third and fourth light reception regions 16C and 16D. The third light reception region 16C generates a signal in accordance with the laser beam irradiated to the region 16C. The fourth light reception region 16D generates a signal in accordance with the laser beam irradiated to the region 16D.
The sub light receiving portion 16S2 is irradiated with the laser beam reflected at the recording surface of the optical disc (or the sub optical spot S2 on the recording surface), whereby the sub optical spot S12 is formed. This sub light receiving portion 16S2 is equally divided into two along a direction corresponding to the track direction and has fifth and sixth light reception regions 16E and 16F. The fifth light reception region 16E generates a signal in accordance with the laser beam irradiated to the region 16E. The sixth light reception region 16F generates a signal in accordance with the laser beam irradiated to the region 16F.
When output signals of the first to sixth light reception regions 16A to 16F are defined as S16A to S16F, by combining these signals, a sum signal RF11 (=S16A+S16B), a tracking error signal TE11 (=S16Axe2x88x92S16B), a first push-pull signal PP11 (=S16Cxe2x88x92S16D), a second push-pull signal PP12 (=S16Exe2x88x92S16F), and a cross track signal CT11 (=PP11xe2x88x92PP12) can be obtained.
FIG. 4 is a schematic waveform diagram illustrating the tracking error signal TE11 and first and second push-pull signals PP11 and PP12 based on the output signals of the light receiving portions 16S0 to 16S2 of FIG. 3B.
The tracking error signal TE11 becomes 0 in signal level when the center of the main optical spot M1 is positioned at the center of a land LA and when it is positioned at the center of a groove GR, while becomes the maximum value or minimum value when the center of the main optical spot M1 is positioned at a border between a land LA and a groove GR.
The first push-pull signal PP11 becomes the minimum value when the center of the main optical spot M1 is positioned at the center of the land LA, while becomes the maximum value when the center of the main optical spot M1 is positioned at the center of the groove GR.
The second push-pull signal PP12 becomes the maximum value when the center of the main optical spot M1 is positioned at the center of the land LA, while becomes the minimum value when the center of the main optical spot M1 is positioned at the center of the groove GR.
FIGS. 5A and 5B are views illustrating the recording surface of an optical disc and the light receiving portions of a photo detector when using the DPP method. In the optical disc drive, for example, the optical pick-up generates a laser beam comprised by the 0-th order diffraction light and the xc2x11st order diffraction lights by the diffraction grating, focuses the laser beams via the object lens to the recording surface of the optical disc to generate three optical spots M3, S3 and S4, and receives the laser beams reflected at the recording surface at the photo detector to generate various signals. The optical spots M3, S3, and S4 move in the radial direction R or the anti-radial direction of the optical disc.
FIG. 5A is a view of the recording surface of an optical disc. Lands LA and grooves GR are formed on the recording surface. The pitch of the lands LA and the pitch of the grooves GR are equal to the track pitch Tp. The center of the main optical spot M3 is positioned at the center of a land LA, and the centers of the sub optical spots S3 and S4 are positioned at centers of grooves GR. The main optical spot M3 is positioned at the middle of the sub optical spots S3 and S4.
Also, the distances between the center of the main optical spot M3 and centers of the sub optical spots S3 and S4 in the disc radial direction R are values obtained by multiplying a half of the track pitch Tp ({fraction (Tp/2)}) by an odd number.
By arranging three optical spots M3, S3, and S4 as shown in FIG. 5A, even when the positional relationship between the object lens and the photo detector changes due to the movement of the object lens, a highly precise tracking error can be obtained without regard as to the displacement of the object lens, that is, the location of the optical spot on the photo detector.
FIG. 5B is a view illustrating the light receiving portions of a photo detector for receiving laser beams reflected at the recording surface.
The laser beams reflected at the recording surface are supplied to light receiving portions 17S0 to 17S2 of the photo detector to thereby generate optical spots M13, S13, and S14. The optical spots M13, S13, and S14 move in the direction Rxe2x80x2 corresponding to the radial direction R when the optical spots M3, S3, and S4 on the recording surface move in the radial direction R of the optical disc.
The main light receiving portion 17S0 is irradiated by the laser beam reflected at the recording surface of the optical disc (or the main optical spot M3 on the recording surface), whereby the main optical spot M13 is formed. This main light receiving portion 17S0 is equally divided into two along a direction corresponding to the track direction and has first and second light reception regions 17A and 17B. The first light reception region 17A generates a signal in accordance with the laser beam irradiated to the region 17A. The second light reception region 17B generates a signal in accordance with the laser beam irradiated to the region 17B.
The sub light receiving portion 17S1 is irradiates by a laser beam reflected at the recording surface of the optical disc (or the sub optical spot S3 on the recording surface), whereby the sub optical spot S13 is formed. This sub light receiving portion 17S1 is equally divided into two along a direction corresponding to the track direction and has third and fourth light reception regions 17C and 17D. The third light reception region 17C generates a signal in accordance with the laser beam irradiated to the region 17C. The fourth light reception region 17D generates a signal in accordance with the laser beam irradiated to the region 17D.
The sub light receiving portion 17S2 is irradiated by the laser beam reflected at the recording surface of the optical disc (or the sub optical spot S4), whereby the sub optical spot S14 is formed. This sub light receiving portion 17S2 is equally divided into two along a direction corresponding to the track direction and has fifth and sixth light reception regions 17E and 17F. The fifth light reception region 17E generates a signal in accordance with the laser beam irradiated to the region 17E. The sixth light reception region 17F generates a signal in accordance with the laser beam irradiated to the region 17F.
When output signals of the first to sixth light reception regions 17A to 17F are defined as S17A to S17F, a tracking error signal TE13 as in the following equation can be obtained by the DPP method. Note that, a coefficient k takes the value in accordance with the ratio of the amount of light between the main optical spot M13 and the sub optical spots S13 and S14.
TE13=S17Axe2x88x92S17Bxe2x88x92k(S17Cxe2x88x92S17D+S17Exe2x88x92S17F) 
Summarizing the disadvantages to be solved by the present invention, in the optical pick-up of U.S. Pat. No. 5,936,923, there is a restriction on the location for providing the tilt sensor. Further, the position at which the laser beam is focused on the optical disc and the position for detecting the tilt by the tilt sensor are different, so it is difficult to correctly detect the tilt of the optical disc.
In recent years, for the purpose of raising the density of the optical disc, a land-groove recording method may be used. In this recording method, a signal is recorded in both of the lands and grooves on an optical disc having a ratio of widths of the lands and grooves of 1:1. In such an optical disc, the high frequency component of the sum signal is small, so it is difficult to detect the cross track signal from the sum signal.
The DPP method is excellent as a method of detection of tracking error. With the conventional DPP method, however, it is difficult to detect the tilt error signal together with the tracking error signal.
Also, when the centers of the three optical spots (one main optical spot and two sub optical spots) are positioned at the centers of lands or when the centers of the three optical spots are positioned at the centers of grooves, if the phase of the push-pull signal of the main optical spot and the phase of the sum signal of the push-pull signals of the sub optical spots are identical, tracking error signals comprised by differential push-pull signals cannot be obtained.
An object of the present invention is to provide an error signal detection device and an error signal detection method for detecting tracking error signals comprised by differential push-pull signals, an optical pick-up useable in the error signal detection device, and a diffraction grating useable in this optical pick-up.
Another object of the present invention is to provide an error signal detection device and an error signal detection method for detecting tracking error signals comprised by differential push-pull signals and a tilt error signal.
According to a first aspect of the present invention, there is provided a diffraction grating for generating a 0-th order diffraction light and xc2x11st order diffraction lights to be irradiated on a recording surface of an optical disc by diffracting a laser beam, wherein each of said xc2x11st order diffraction lights irradiated to said optical disc has a phase distribution equivalent or substantially equivalent to a wavefront aberration of said optical disc when there is a tilt in said optical disc, distances between a main optical spot corresponding to said 0-th order diffraction light and sub optical spots corresponding to said xc2x11st order diffraction lights in a disc radial direction at said recording surface are identical or substantially identical to a whole multiple of a pitch of tracks or track guide grooves of said optical disc, and a phase of a push-pull signal of said 0-th order diffraction light and a phase of a sum signal of the push-pull signals of said xc2x11st order diffraction lights are different from each other.
Preferably, said main optical spot and said sub optical spots are formed on an identical track of said recording surface.
Preferably, one of said xc2x11st order diffraction lights has a phase distribution equivalent or substantially equivalent to the wavefront aberration in a case when a tilt angle of said optical disc is a positive constant angle, and the other of said xc2x11st order diffraction lights has a phase distribution equivalent or substantially equivalent to the wavefront aberration in a case when the tilt angle of said optical disc is a negative constant angle.
In the diffraction grating according to the present invention, for example, said wavefront aberration may be coma occurring at a transparent substrate of said optical disc.
In the diffraction grating according to the present invention, for example, said optical disc has an address region with pits indicating the address formed therein and a data region with lands and grooves formed therein. Said pits and said lands may be arranged on the tracks of said recording surface.
According to a second aspect of the present invention, there is provided an optical pick-up comprising a laser for outputting a laser beam, a diffraction grating for generating a 0-th order diffraction light and xc2x11st order diffraction lights by diffracting the laser beam from said laser, an object lens for focusing said 0-th order diffraction light and xc2x11st order diffraction lights and irradiating the same to the recording surface of the optical disc to form a main optical spot corresponding to said 0-th order diffraction light and sub optical spots corresponding to said xc2x11st order diffraction lights on said recording surface, and a photo detector provided with a main light receiving portion for receiving said 0-th order diffraction light reflected at said recording surface and sub light receiving portions for receiving said xc2x11st order diffraction lights reflected at said recording surface, wherein each of said xc2x11st order diffraction lights irradiated to said optical disc has a phase distribution equivalent or substantially equivalent to a wavefront aberration of said optical disc when there is a tilt in said optical disc, distances between the center of said main optical spot and centers of said sub optical spots in a disc radial direction on said recording surface are identical or substantially identical to a whole multiple of a pitch of tracks or track guide grooves, and the phase of a push-pull signal of said 0-th order diffraction light based on the output signal of said main light receiving portion and the phase of the sum signal of the push-pull signals of said xc2x11st order diffraction lights based on the output signals of said sub light receiving portions are different from each other.
Preferably, said main optical spot and said sub optical spots are formed on the identical track of said recording surface.
Preferably, one of said xc2x11st order diffraction lights has a phase distribution equivalent or substantially equivalent to the wavefront aberration when the tilt angle of said optical disc is a positive constant angle, and the other of said xc2x11st order diffraction lights has a phase distribution equivalent or substantially equivalent to the wavefront aberration when the tilt angle of said optical disc is a negative constant angle.
In the optical pick-up according to the present invention, for example, said wavefront aberration is coma occurring at a transparent substrate of said optical disc, and said main light receiving portion and sub light receiving portions are configured divided along a direction corresponding to the radial direction of said optical disc.
In the optical pick-up according to the present invention, for example, said optical disc has an address region with pits indicating the address formed therein and a data region with lands and grooves formed therein. Said pits and said lands can be arranged on the track of said recording surface.
According to a third aspect of the present invention, there is provided an error signal detection device comprising a laser for outputting a laser beam, a diffraction grating for generating a 0-th order diffraction light and xc2x11st order diffraction lights by diffracting the laser beam from said laser, an object lens for focusing said 0-th order diffraction light and xc2x11st order diffraction lights and irradiating the same to the recording surface of the optical disc to form a main optical spot corresponding to said 0-th order diffraction light and sub optical spots corresponding to said xc2x11st order diffraction lights on said recording surface, a photo detector provided with a main light receiving portion for receiving said 0-th order diffraction light reflected at said recording surface and sub light receiving portions for receiving said xc2x11st order diffraction lights reflected at said recording surface, and a generation circuit for generating a main push-pull signal corresponding to said 0-th order diffraction light and first and second sub push-pull signals corresponding to said xc2x11st order diffraction lights based on output signals of said photo detector and generating tracking error signals as differential push-pull signals obtained by subtracting a sum signal of said first and second push-pull signals from said main push-pull signal, wherein each of said xc2x11st order diffraction lights irradiated to said optical disc has a phase distribution equivalent or substantially equivalent to a wavefront aberration of said optical disc when there is a tilt in said optical disc, distances between the center of said main optical spot and centers of said sub optical spots in a disc radial direction on said recording surface are identical or substantially identical to a whole multiple of a pitch of tracks or track guide grooves, and the phase of said push-pull signal and the phase of the sum signal of said first and second sub push-pull signals are different from each other.
Preferably the device further has a tilt detection circuit for generating a tilt error signal corresponding to a tilt of said optical disc, said generation circuit generates a main reproduced signal corresponding to an amount of reflected light of said 0-th order diffraction light and first and second sub reproduced signals corresponding to amounts of reflected light of said xc2x11st order diffraction lights based on output signals of said photo detector, and said tilt detection circuit generates said tilt error signal based on a difference of amplitudes of alternating current components of said first and second sub reproduced signals.
More preferably, said tilt detection circuit comprises a first high pass filter for extracting the alternating current component of said first sub reproduced signal, a first envelope detector for detecting the envelope of the signal extracted by said first high pass filter, a second high pass filter for extracting the alternating current component of said second sub reproduced signal, a second envelope detector for detecting the envelope of the signal extracted by said second high pass filter, and a subtractor for generating said tilt error signal based on a difference of amplitudes of output signals of said first and second envelope detectors.
More preferably, said tilt detection circuit further has a first low pass filter for extracting a direct current component of said first sub reproduction signal, a first divider for dividing the amplitude of the output signal of said first envelope detector by the direct current component extracted by said first low pass filter, a second low pass filter for extracting the direct current component of said second sub reproduction signal, and a second divider for dividing the amplitude of the output signal of said second envelope detector by the direct current component extracted by said second low pass filter, and said subtractor generates said tilt error signal based on the difference of the output signals of said first and second dividers.
Preferably, said optical disc has an address region with pits indicating an address formed therein and a data region with lands and grooves formed therein, said pits and said lands are arranged on the track of said recording surface, said generation circuit detects said tracking error signal of said data region, and said tilt detection circuit detects said tilt error signal of said address region.
Preferably, the width of said lands and the width of said grooves are identical or substantially identical.
Preferably, said generation circuit generates a cross track signal based on the difference between said first and second sub push-pull signals.
Preferably, said main optical spot and said sub optical spots are formed on the identical track of said recording surface.
Preferably, one of said xc2x11st order diffraction lights has a phase distribution equivalent or substantially equivalent to the wavefront aberration in the case when the tilt angle of said optical disc is a positive constant angle, and the other of said xc2x11st order diffraction lights has a phase distribution equivalent or substantially equivalent to the wavefront aberration in the case when the tilt angle of said optical disc is a negative constant angle.
In the error signal detection device according to the present invention, for example, said wavefront aberration is coma occurring at the transparent substrate of said optical disc, and said main light receiving portion and sub light receiving portions are configured divided along a direction corresponding to the radial direction of said optical disc.
According to a fourth aspect of the present invention, there is provided an error signal detection method comprised of a step of generating a 0-th order diffraction light and xc2x11st order diffraction lights by diffracting a laser beam and irradiating the generated 0-th order diffraction light and xc2x11st order diffraction lights to a track of a recording surface of the optical disc, a step of generating a main push-pull signal corresponding to said 0-th order diffraction light reflected at said recording surface and first and second sub push-pull signals corresponding to said xc2x11st order diffraction lights reflected at said recording surface, and a step of generating a tracking error signal as a differential push-pull signal obtained by subtracting a sum signal of said first and second sub push-pull signals from said main push-pull signal, wherein each of said xc2x11st order diffraction lights irradiated to said optical disc has a phase distribution equivalent or substantially equivalent to a wavefront aberration of said optical disc when there is a tilt in said optical disc, distances between the center of said main optical spot and centers of said sub optical spots in a disc radial direction on said recording surface are identical or substantially identical to a whole multiple of a pitch of tracks or track guide grooves, and the phase of said push-pull signal and the phase of the sum signal of said first and second sub push-pull signals are different from each other.
The error signal detection method according to the present invention is preferably further comprising a step of generating a main reproduced signal corresponding to an amount of reflected light of said 0-th order diffraction light reflected at said recording surface and first and second sub reproduced signals corresponding to amounts of reflected light of said xc2x11st order diffraction lights reflected at said recording surface and a step of generating a tilt error signal corresponding to a tilt of said optical disc based on a difference of amplitudes of alternating current components of said first and second sub reproduced signals.
Preferably, said optical disc has an address region with pits indicating the address formed therein and a data region with lands and grooves formed therein, said pits and said lands are arranged on tracks of said recording surface, and in the step of generating said tracking error signal, said tracking error signal of said data region is generated, and in the step of generating said tilt error signal, said tilt error signal of said address region is generated.
Preferably, the width of said lands and the width of said grooves are identical or substantially identical.
The error signal detection method according to the present invention is preferably further comprised by a step of generating a cross track signal based on a difference between said first and second sub push-pull signals.
Preferably, in said step of irradiation, the main optical spot corresponding to said 0-th order diffraction light and the sub optical spots corresponding to said xc2x11st order diffraction lights are formed on the identical track of said recording surface.
Preferably, one of said xc2x11st order diffraction lights has a phase distribution equivalent or substantially equivalent to the wavefront aberration when the tilt angle of said optical disc is a positive constant angle, and the other of said xc2x11st order diffraction lights has a phase distribution equivalent or substantially equivalent to the wavefront aberration when the tilt angle of said optical disc is a negative constant angle.
Preferably, the xc2x11st order diffraction lights generated by the diffraction grating have phase distributions equivalent or substantially equivalent to the wavefront aberration of the optical disc when there is a tilt in the optical disc.
Distances between the center of the main optical spot and the centers of the sub optical spots in the disc radial direction at the recording surface of the optical disc are set at whole multiples of the track pitch or whole multiples of the pitch of the track guide grooves. Also, the phase of the push-pull signal of said 0-th order diffraction light and the phase of the sum signal of the push-pull signals of said xc2x11st order diffraction lights are different from each other. Accordingly, it is possible to obtain a tracking error signal comprised by the differential push-pull signals.
The magnitude of the amplitude of the alternating current component of the first sub reproduced signal corresponding to the amount of the reflected light of one of the xc2x11st order diffraction lights becomes the maximum when there is a predetermined tilt angle (+xcex8) and monotonously becomes smaller the further from the related tilt angle (+xcex8).
The magnitude of the amplitude of the alternating current component of the second sub reproduced signal corresponding to the amount of reflected light of one of the xc2x11st order diffraction lights becomes the maximum when there is a predetermined tilt angle (xe2x88x92xcex8) and monotonously becomes smaller the further from the related tilt angle (xe2x88x92xcex8).
The difference of amplitudes of the alternating current components of these sub reproduced signals corresponds to the tilt angle of the optical disc. The tilt error signal can be detected based on the related difference of amplitude. Also, even in the case where the tilt angle of the optical disc is near 0 degree, the tilt angle can be correctly detected, and the detection precision of the tilt of the optical disc can be improved.